1. Field of the Invention
This invention pertains generally to structural components for vacuum enclosures and particularly to structures having specific features for reducing secondary electron emission and sputtering from the walls of a vacuum enclosure and having specific features for sorbing gases.
2. Description of the Prior Art
Many devices make use of the flow of molecular atomic or subatomic particles in a controlled environment. The environment may be a vacuum or a known pressure of desired gases depending upon the function required of the particular device. The particles may be electrons or electrically charged ions or molecules. These devices are usually associated with means for accelerating the particles such as a system of electrodes whose potentials are known. Frequently use is also made of magnetic fields.
Whatever the nature of the particles may be they are usually in motion and so possess a kinetic energy.
In some cases, in order to perform their desired function, the primary particles are caused to impinge upon a target. For instance in the case of a thermionic valve electrons emitted from a cathode are accelerated by an electric potential thus gaining kinetic energy and eventually are collected upon an anode, where upon the kinetic energy of the electrons is at least partially transformed into other forms of energy.
In other cases the particles may deviate from their intended path and impinge upon surfaces within the device upon which they are not intended to impinge. Such is often the case in devices known as particle storage devices or accelerators such as cyclotrons, betatrons etc. Furthermore the controlled beam of particles may collide with molecules or atoms of the residual gas atmosphere of the device causing these molecules or atoms to undesirably impinge upon surfaces within the device.
When a particle impinges upon a surface several phenomena may occur depending upon the kinetic energy and nature of the particle and the surface. The kinetic energy of the particle may be transformed into vibrations of the atomic lattice constituting the impacted surface and thus manifests itself as heat. The energy of the particle may be transfer red to only one or a few of the atoms of the impacted surface lattice in which case these atoms may become detached from the surface. Such detached atoms can upon other surfaces within the device. This phenomenon known as sputtering is usually undesirable. The impinging particle may cause the surface to re-emit charged particles such as in the well known effect of secondary electron emission. Again such secondary emission is very often undesirable. Alternatively the particles may simply be reflected thus a surface which, intentionally or unintentionally, is impinged upon by particles can cause undesirable effects.
In patent application Ser. No. 539,101, filed Jan. 7, 1975 there are described charged particle collecting bodies or traps comprising a three dimensional network defining a multiplicity of inter-connecting free cells such that a large percentage of the charged particles, incident upon the surface defining said network pass through said surface without impinging upon the material constituting said network. Said network allows at least part of said percentage of charged particles to impinge upon the material of said network at a position below the surface defining said network. Thus secondary electrons produced below the surface find difficulty in escaping from said surface and tend to be captured by collision with the surrounding network.
In practice such charged particle collecting bodies have to be carefully machined or formed to shape before being located in their desired position. Difficulties can be encountered in rigidly attaching the particle collecting body within the vacuum enclosure due to differences in thermal expansion coefficients of the materials used to make the vacuum vessel walls and the particle collecting bodies. Attachment by means of bolts or similar devices can strain the enclosure walls and in extreme cases could lead to loss of integrity of the vacuum enclosure, or deformation of the particle collecting body.
A further difficulty in many vacuum enclosures is the production and maintenance of a suitable degree of vacuum. In large vacuum enclosures such as particle accelerators many vacuum pumps are required, distanced around the enclosure. Never the less in the space between two pumping appertures within the enclosure there may manifest itself a relatively high pressure region of gases desorbed from the enclosure walls due to the distance separating that region from the nearest pump, even though the pump may be in continuous operation during normal working of the vacuum device comprising the enclosure. In other vacuum enclosures it may not be desirable to operate the pumps after initial creation of the desired vacuum. It is then difficult to ensure maintenence of this vacuum during operation of the vacuum device comprising the vacuum enclosure.
It is therefore an object of the present invention to provide a wall structure for a vacuum enclosure which is substantially free from one or more of the defects of previously known walls of vacuum enclosures.
Another object of the present invention is to provide a wall structure for a vacuum enclosure which is substantially free from sputtering.
Another object of the present invention is to provide a wall structure for a vacuum enclosure which is substantially free from secondary electron emission.
A further object of the present invention is to provide a wall structure for a vacuum enclosure which is capable of sorbing gases.
Further objects and advantages of the wall structure for a vacuum enclosure of the present invention will be obvious to those skilled in the art from the following detailed description thereof taken in conjunction with the accompanying figures.